Epoxides, including propylene oxide, butylene oxide, epichlorohydrin, and the like, are widely used precursors for the production of other compounds. Most epoxides are formed via halohydrin intermediates, and these processes are well known to those skilled in the art, such as disclosed in U.S. Pat. No. 5,532,389 and British Patent No. 2,173,496. The halohydrins are most often reacted with an aqueous alkali stream to produce epoxides and the subsequent halide salt. The epoxide-water azeotrope is advantageously stripped from the aqueous stream to minimize by-product losses resulting from the reaction of water with epoxide to form glycols, such as ethylene glycol, propylene glycol, 3-chloro-1,2-propandiol, glycidol, and glycerine. This overhead product, including water and epoxide, is then condensed and separated in a liquid-liquid phase separator to form an aqueous fraction and an organic fraction containing the crude epoxide, which may be further purified. The aqueous fraction from the overhead is returned to the distillation column as reflux. The hydrolysis of the epoxide can be further enhanced by the presence of either acid or base, and is reduced at a pH of 7. The process of ring closure by the action of an alkali is also described by the terms epoxidation, saponification, and in the case of the halogen chlorine, dehydrochlorination.
In industrial processes, halohydrins are made by reacting low molecular weight olefin-containing compounds, such as propylene, butylene, and allyl chloride, with chlorine (or other halogens) and water in a reaction referred to as hypochlorination. Thus propylene and butylene are converted to chlorohydrins and allyl chloride to dichlorohydrins, and subsequently to their respective epoxides (e.g., propylene oxide, butylene oxide, and epichlorohydrin). This process produces both isomers of the halohydrins, and the resulting halohydrins are often dilute in water (<10% by weight) and contain an equivalent of haloacid from the reaction. The halohydrin stream produced by hypohalogenation may then be fed directly to a reactive distillation column with an alkali, or first, to a pre-reactor for neutralization of the haloacid and partial conversion of the halohydrin before introduction into the reactive distillation column. For example, Japanese Patent No. JP 1994-025196(B2) discloses a process where dilute dichlorohydrins are mixed with Ca(OH)2 at 40° C. in a pre-reactor and then fed to a reactive distillation column where the epoxide (epichlorohydrin) is stripped overhead with water and phase separated from the water in the overhead phase separator to obtain epichlorohydrin.
Another technology, used to a lesser extent in industry, is the reaction of glycols with HCl and a carboxylic acid catalysis to produce the halohydrin, such as disclosed in U.S. Patent Application Publication No. 20080015370. As described therein, mostly one isomer of the halohydrin (1,3-dichlorohydrin) is produced, and the remainder of the stream contains less than 30% water and less than 10% HCl, each by weight. This halohydrin stream is fed with a 10% NaOH stream to a reactive distillation column where epichlorohydrin is stripped overhead with water and phase separated from the water in the overhead phase separator to obtain epichlorohydrin.
A third technology, used specifically for the production of epichlorohydrin, is the catalytic acetoxylation of propylene into allyl acetate, hydrolysis of the allyl acetate into allyl alcohol, and catalytic chlorination of the allyl alcohol into dichlorohydrin, as disclosed in U.S. Pat. No. 4,634,784. As disclosed therein, mostly one isomer of the halohydrin (2,3-dichlorohydrin) is produced, and the remainder of the stream contains less than 20% water and 5% HCl, each by weight. This halohydrin stream is fed with a 9.5% Ca(OH)2 slurry to a column where epichlorohydrin is stripped overhead with water and phase separated from the water in the overhead phase separator to obtain epichlorohydrin.
Three main reactions occur during the process to convert halohydrins to epoxides: neutralization of the haloacid, dehydrohalogenation of the halohydrin, and the hydrolysis of the epoxide to glycol. The dehydrohalogenation of the halohydrin, for example, may be performed with an alkali. The halohydrin can be dilute in aqueous or mostly organic stream, and often consists of two isomers as well as haloacid. The base is typically an aqueous stream or slurry consisting of NaOH or Ca(OH)2, with or without the presence of a salt, such as NaCl and CaCl2. In order to avoid yield losses of the epoxide to hydrolysis, the epoxide is often stripped during the reaction in a distillation column and pH is maintained as close to neutral as possible, as the hydrolysis rate is catalyzed by both acid and base. The glycols produced, and some residual organics, are not strippable and are lost in the aqueous stream with the salt formed, which exits the bottom of the distillation column and constitute the major yield loss from the dehydrohalogenation process. The bottom aqueous stream may be treated before discharge or recycle. Thus, hydrolysis losses not only impact epoxide yield, but also wastewater treatment cost and capital investment.
A wide variety of embodiments of processes and apparatus for the dehydrohalogenation of mostly organic halohydrins have been proposed in the prior art. For example, Russian Patent No. 2,198,882 disclose mixing an anhydrous dichlorohydrin stream, distilled from the dilute stream containing a mixture of dichlorohydrin isomers from hypochlorination of allyl chloride with chlorine and water, with 28% NaOH in a continuous stirred tank reactor (CSTR) to produce epichlorohydrin, which is then subsequently stripped. This system is biphasic, with an organic dichlorohydrin phase and an aqueous phase with the NaOH. As all reactions occur primarily in the aqueous phase, mass transfer is a major factor to obtaining high epoxide yields. U.S. Pat. No. 4,496,753 discloses a similar biphasic system, with the dichlorohydrins in an organic solvent (CCl4) in a 2-stage reactor with a CSTR followed by a plug flow reactor (PFR). However, these methods have technical and economical drawbacks. One such drawback is the need for additional equipment, such as a CSTR, and another drawback is that backmixing in the CSTR exposes epoxide to hydrolysis with the incoming acid and base, depending on the intensity of mixing. In the case with a solvent, an addition distillation column for solvent recovery is needed, and depending on the partitioning of the epoxide between the two phases, may also lead to additional epoxide losses.
U.S. Pat. No. 4,634,784 (the '784 patent) discloses catalytic acetoxylation of propylene into allyl acetate, hydrolysis of the allyl acetate into allyl alcohol, catalytic chlorination of the allyl alcohol into dichlorohydrin, and the dehydrochlorination of the dichlorohydrin into epichlorohydrin. Predominately only the 2,3-dichlorohydrin isomer is produced in a 75% by weight organic stream. This system is biphasic, with an organic dichlorohydrin phase and an aqueous phase or slurry with the base. As all reactions occur primarily in the aqueous phase, mass transfer is a major factor to obtaining high epoxide yields. Several methods of dehydrochlorination are described: the dichlorohydrin and milk of lime is (1) fed directly to the top of a distillation column, (2) mixed while stirring, and (3) reacted in an inert solvent insoluble to water. The disadvantages of methods (2) and (3) have already been discussed above. For method (1), the direct feeding to a distillation column, the mixing intensity is not as great on the plates in the distillation column as in the cases with CSTR, resulting in lower conversions. The example given in the '784 patent shows a 2,3-dichlorohydrin stream fed with a 9.5% Ca(OH)2 slurry to a column to obtain a conversion of 88% and a selectivity of 97%. The low dichlorohydrin conversion dictates larger recovery equipment, higher recycle, and higher cost.
WO 2006/020234 discloses the reaction of glycols with HCl and carboxylic acid catalysis to produce the halohydrin, particularly for glycerine to dichlorohydrins to epichlorohydrin. The dichlorohydrin produced is predominately the 1,3-dichlorohydrin isomer, which has a much faster dehydrochlorination reaction rate than the 2,3-dicholohydrin isomer. The base used can be NaOH or Ca(OH)2 and is preferably 20-60% by weight. This system is biphasic, with an organic dichlorohydrin phase and an aqueous phase with the base. As all reactions occur primarily in the aqueous phase, mass transfer is a major factor to obtaining high epoxide yields. No mention is made of importance of mixing intensity and high specific interfacial surface area as the reaction is mass transfer limited. WO 2006/020234 gave an example of feeding the dichlorohydrin with 10% NaOH to a distillation column and obtained good product quality with no mention of yield.
Accordingly, there exists a need for improved processes and apparatus for the dehydrohalogenation of halohydrins in which the overall by-product hydrolysis reaction may be reduced in order to obtain good epoxide selectivity and conversion.